This invention relates to a process for heating and refining molten materials, and more specifically, to a process for heating and refining molten materials by subsurface injection of oxygen and a fluid fuel.
Molten metal refining processes known in the art have utilized subsurface injection of pure oxygen and a fluid hydrocarbon fuel. In some metal refining processes, the oxygen and hydrocarbon have been injected through a "shroud"-type tuyere. In these instances the oxygen is injected through a central tube and the hydrocarbon is injected through a surrounding annular tube, thereby forming a shroud around the oxygen. This shroud serves a protective function to prevent excessive erosion of the tuyere and surrounding refactory by the oxygen. The most widespread commercial use of this concept has been in Q-BOP steelmaking, described in U.S. Pat. No. 3,930,843. Applications in copper smelting have also been disclosed, for example, in U.S. Pat. Nos. 3,990,889 and 3,990,890.
While the hydrocarbon does play a part in the above process reactions, these processes are essentially oxidation refining operations. The amount of hydrocarbon injected is small in relation to the amount of oxygen injected (only up to about 8% in Q-BOP steelmaking) so as not to impede oxidation of the molten metal impurities. The low level of hydrocarbon protects the tuyere and refractory and, in doing so, results in the formation of frozen accretions. As used herein, the term "frozen accretions" refers to a formation of solid metal and/or slag in a molten metal bath near a tuyere which forms as the result of the cooling effect of an injected fluid. A frozen accretion is known variously in the art as a knurdle, a shanker, and a mushroom cap. As fluid injection proceeds, frozen accretions tend to increase in size until a thermal equilibrium is reached. Accretions can grow in size to cause blockage of a tuyere opening. For this reason, it has been heretofore thought that relatively high amounts of hydrocarbon could not be injected as a shroud around oxygen without encountering accretion problems.
Oxygen and hydrocarbons have also been utilized in refining copper, and, specifically, anode grade copper. Anode grade copper is refined from ore in a series of steps before it is ready for casting into anodes or other products. The initial steps of beneficiation, smelting and converting serve to concentrate and purify the ore to product crude or "blister" copper. The final refining step (known in the art as "fire refining") accomplishes the reduction of oxygen and sulfur impurities in the blister copper, typically from levels of 0.70% and 0.05%, respectively, to levels below 0.20% and 0.005%, respectively. Copper remelted from scrap may be fire refined also, either together with virgin material or by itself.
Fire refining is usually carried out in the temperature range of about 2000.degree. F. (1090.degree. C.) to 2200.degree. F. (1200.degree. C.) and in two steps. In the first step, an oxygen containing gas is injected beneath the surface of a bath of molten blister copper to oxidize sulfur to sulfur dioxide, which thereafter floats up and out of the bath. In the second step, known in the art as "poling", dissolved oxygen in the molten copper is removed by reduction with a hydrocarbon. The term "poling" comes from the traditional practice of immersing green wood poles in the molten bath to supply the fuels. More recent innovations in fire refining include the direct injection of mixtures of oxygen-containing gas and hydrocarbon fuels into the bath. The direct injection of these mixtures, generally by means of tuyeres located below the surface of the molten copper, has made it possible to control the fire refining process to a greater degree. This added control has not been without some degree of danger due to the presence of explosive mixtures of fluids in the piping.
The hydrocarbon fuels injected into the molten copper crack to produce carbon and hydrogen, which thereafter react with oxygen to form carbon monoxide, carbon dioxide, and water. These are emitted as off-gas from the molten copper bath. During the poling step, unreacted hydrocarbons may be emitted from the bath, as well as carbon soot formed from the incomplete combustion of the hydrocarbons.
Reduced opacity of emissions has become a major goal of commercial copper refiners. "Opacity", as used herein, refers to the capacity of the off-gas to obstruct the transmission of light, expressed as a percentage. No obstruction is expressed as 0%, while total obstruction is expressed as 100%. Volatile hydrocarbons, carbon soot, and other particulates emitted from molten copper baths during fire refining are major causes of emissions of high opacity from copper refining plants. Previous methods of fire refining copper have relied on post-treatment of the off-gases from the molten copper to meet opacity limits, now restricted to 20% or less in some cases. In the case of solid particulate matter, conventional baghouses are used to trap escaping matter. Volatiles on the other hand are removed by utilizing complex and costly afterburners, cooling towers and other systems to remove them from the off-gas.
Improved deoxidation efficiency has also become an important goal of commercial copper refiners. As used herein, "deoxidation efficiency" refers to the ratio, expressed as a percentage, of the actual amount of oxygen removed from the molten metal bath (impurity plus injected oxygen) per unit of fuel injected, to the theoretical amount of oxygen required to completely react with a unit of the fuel. While some high deoxidation efficiencies have been reported in relatively small-scale tests, the deoxidation efficiencies of commercial size reactors (1-150 tons and higher) have remained low. Improvement in this area brings the obvious benefit of lower fuel expenditures per unit of copper refined.
Conventional heating and refining processes have performed inefficiently due to low heat recovery. As used herein, "heat recovery" refers to the ratio, expressed as a percentage, of the sum of the amount of heat given off from the furnace to its environment plus the amount of heat absorbed during the process in raising the molten bath temperature, to the theoretical heat of combustion available from the injected fuel. In equation form, this is expressed as follows: ##EQU1## Where A=temperature rate increase of bath (.degree.F./min) (.degree.C./min)
B=heat capacity of bath (Btu/.degree.F.) (cal/.degree.C.) PA1 C=heat loss of furnace (Btu/min) (cal/min) PA1 D=fuel fuel flowrate (ft.sup.3 /min)(M.sup.3 /min) PA1 E=heat of combustion of fuel (Btu/ft.sup.3) (cal/m.sup.3)
This inefficiency has been especially apparent in the copper industry, where additional external heat input has been necessary to melt solid copper, usually prior to the refining step. Solid copper has also been added as a means of cooling down the bath when the bath temperature has exceeded the conventional fire refining range of 2000.degree. F. (1090.degree. C.) to 2200.degree. F. (1200.degree. C.). The recovery of available heat self-generated by the reaction of the impure molten copper and injected materials in prior fire refining processes has not been sufficient to overcome the cooling effect of solid copper additions to the bath at conventional fire refining temperatures.
The following patents disclose fire refining of impure molten copper by the injection of hydrocarbon fuels and oxygen-containing gas.
U.S. Pat. No. 3,258,330 discloses a process for fire refining blister copper wherein air containing oxygen in various densities is mixed with a solid or liquid hydrocarbon fuel and injected into a molten copper bath during the heating, oxidation and reduction stages of refining. The preferred ratios of oxygen to hydrocarbon, in terms of the theoretical amount necessary for combustion, are 80% to 130% during heating, 100 % to 200 % during oxidation, and 20% to 100% during reduction. The deoxidation efficiencies calculated from the patent disclosure range from about 30 to 40%.
U.S. Pat. No. 3,619,177 discloses a process for reducing the oxygen content of molten copper during fire-refining by introducing a mixture of a gaseous hydrocarbon and either air, oxygen-enriched air, or pure oxygen through a single tuyere below the bath surface in a quantity sufficient to form a reducing gas mixture within the melt. The calculated deoxidation efficiencies were 46 to 93% in small scale tests (up to 939 lbs. of molten copper), while in plant-scale testing (215 to 325 tons of molten copper), calculated deoxidation efficiency dropped to a range of 31 to 35%. The patent further discloses that pollutants emitted from the molten copper bath are minimized by blowing air and creating a reducing gas mixture over the bath.
Bearing in mind these and other deficiencies of the prior art, it is therefore an object of the present invention to provide a process for efficiently heating molten materials.
It is another object of the present invention to provide a process for refining impure copper which reduces air pollution.
It is another object of the present invention to provide a process for refining impure copper with increased deoxidation efficiency.
It is further object of the present invention to increase the heat recovery in a fire refining process.
It is another object of the present invention to utilize solid copper in a fire refining process without additional external heat input.
It is still another object of the present invention to provide a heating and refining process which is relatively free of the formation of tuyere blocking accretions.